This is an interesting contest. 74 entrants were winnowed down to 4, and now to the final 1, at least assuming Wandsworth Council accepts the jury panel's recommendation (which seems to be a formality). However, it's not a contest to choose a bridge design: it's a contest to choose a bridge design team, and it's possible that once they are appointed, and enter into further discussions with key stakeholders, an entirely different design could emerge.

I covered the four shortlisted designs, including Bystrup's, previously, so I won't go over the design's merits in detail again. Suffice to say that it's key feature is cycle ramps suspended above the River Thames, protecting river bank amenities (especially a public gardens on the north bank) at the expense of taking over large areas of the river channel.

Assuming Wandsworth Council accept their jury's decision, the next step is to formally appoint the design team and begin further design development. It may therefore be some time before we hear much more of the project.

The scheme seems to remain unpopular, especially with residents on the Pimlico side of the river. The Guardian noted "public fury" over the scheme but points out that much of the anger is misplaced, as the impact on the river bank park is actually minimal.

09 December 2015

In a recent post, I reviewed Bridging the Dutch Landscape, a sort of how-to-design-bridges-for-beginners produced several years ago by the design firm ipv Delft.

In 2014, ipv Delft helped prepare a new bridge design guide for the Dutch infrastructure knowledge-sharing body, CROW. Now, this has been translated into English in abbreviated form. You can download a free copy; I'm not sure whether any hard copies are still available.

As with Bridging the Dutch Landscape, this is a well-illustrated volume, with plenty of diagrams and photographs as examples. The Brief Dutch design manual is, however, a little more technical than its predecessor, more concerned with the requirements that a designer must satisfy than with the opportunities available to them.

There is a great deal of good material on spatial requirements for different types of pedestrians, and cyclists. I was interested in the contrast between this and the UK footbridge design standard, BD29, which is something of a blunt instrument, intended for simple urban bridges and often applied in situations where it is unhelpfully restrictive.

The Dutch approach takes greater account of different users, and allows a little more flexibility in how the width of a bridge is determined and apportioned. As you might expect, there is plenty of advice on spatial requirements for cyclists, and even a mention for designing for the needs of in-line skaters! This information extends to a more coherent approach to ramp gradients, with a concept of "difficulty" which takes account of not only the gradient but also the total length of ramp, such that over a short distance, a steeper ramp may be deemed acceptable. This offers the designer the chance to interpret a given situation, rather than simply respond to hard-edged rules.

There are other areas, however, where the Dutch design standards are strangely onerous, such as their challenging requirements for collision loads on bridges spanning a highway, and also their parapet loadings, which are also inexplicably large. Against these can be set another example of context-sensitive specification, for the space between elements in a parapet railing. In the UK, this is normally set at a blanket gap size of 100mm, but the Dutch permit a gap as large as 500mm in some cases, only requiring tighter limits in cases where the bridge is likely to be used frequently by young children.

As with its predecessor, the book has some brief but useful information on actual costs of real footbridges, and also a handful of project examples, such as Eindhoven's Hovenring.

I can see myself referring to this design manual any time I'm designing a bridge for cyclists, or when I want to try and challenge a client to see beyond the standard rules and regulations. It's a useful reminder that a designer can apply their own judgement in response to a specific situation, and that the normal regulatory constraints are essentially arbitrary and open to debate.

The winner is Marc Mimram with Webb Yates (the latter were listed as part of the shortlisted team, but curiously aren't acknowledged on the BANES website). I'd be interested to see a jury report or other details of how the winner was chosen, as BANES had initially said the choice would be on a combination of design quality and fee proposal. There's a complete lack of transparency around the entire competition process.

The winning design has echoes of Mimram's Pont de Solferino in Paris, which shares the vierendeel truss ribbed girders. The Paris bridge is a marvel of complex craftsmanship, but the complexity of its detailing led French bridge expert Michel Virlogeux to comment that "fantasy governed the detailed design, a fantasy which had not been tempered by the rationality of a serious engineer".

Mimram's design for Bath follows an S-shaped curve in plan, and coupled with the sinuous variation in girder height, this means that no two pieces of metalwork will be the same.

The designer suggests that the bridge "adapts to the scheme of stresses", by which he essentially means that the girder depth follows the bending moment diagram for a two-span bridge with one short and one long span. On closer examination of the elevation, this is, however, not actually the case. It's broadly true for the longer span, but it isn't true for the short side-span, which is deepest half-way along rather than over the pier, which is where the forces are greatest. This is a consequence of having the footway deck follow the girder chord - the "correct" girder shape would have a kink in it, which is clearly visually unacceptable here.

The pier supports the bridge not directly from the edge girders, as would make most structural sense, but from a hammerhead cross-beam. That leaves the visual emphasis on the bridge's sensual shape, but in my view also leaves it looking strangely incomplete. The hammerhead as shown doesn't seem deep enough at its connection to the main girders to carry all the loads properly.

It will be interesting to see how the project develops. £2.5m is an ample budget for a roughly 60m long footbridge, but Mimram's approach to detailing his design must be one of the biggest potential sources for a cost over-run.

Setting aside the structural behaviour, I think it's an appropriately modest design, which could offer a pleasant promenade over the river.

One of the potential problems with a shortlisted competition is that if any of the entrants come up with a design which is in any way unacceptable, significant effort may be wasted and the promoter's pool of options can be drastically reduced. Many of the designs shown here have significant issues to overcome, and it's not clear to me that any of these designs is entirely fit for purpose.

I am strangely reminded of the hallucigenia, a bizarre prehistoric creature which confused many researchers, who initially depicted it upside down. This is something of an upside-down bridge, a steel box-girder installed with a slight upwards arch, and then pulled downwards by a series of stainless steel cables. At first sight, it's as if the bridge is filled with helium and needs to be tied down to prevent it floating away.

The design entry indicates that the cables induce prestress in the bridge deck, although in pure bending terms it is the opposite of a helpful prestress. I can only imagine that the deck is placed into axial compression by virtue of its fixed end points, and this helps counteract normal bending forces. However, it would require a very heavy upper plate to prevent buckling, and has the unfortunate property that the more load is added, the less effective the prestress becomes. I struggle to get my head around the whole idea, and it's not helped by deeply unconvincing cable anchorage details.

The design has a painted steel girder, not ideal for maintenance above such a steep gorge. The parapets are lightweight and open, which will fail to reassure many bridge users, and worse, the deck is an aluminium grille, which will terrify the rest. I find it hard to see how this is an inclusive design suitable for all users, particularly the disabled or infirm.

This second design is also a steel box girder bridge, although very different in style. The cross-section is intended to give the impression that the girder is very slender, like the blade of an Arthurian sword, but at heart it's a conventional design, a continuous steel girder on concrete supports.

The supports are "striated" with concrete layers, an hommage to slate and mudstone geology, but are cast as a series of infilled precast shells. This seems a somewhat forced approach, as the precast shell has to be cast in multiple stages only to hide a monolithic core.

The balustrades seem slightly more secure than the previous design, but the most interesting feature of this bridge is that the girders are constructed of roll-bonded structural steel, a material more generally used in cladding and industrial vessels, and which I've not seen used for a bridge like this. Conventional structural steel would be hot-pressed against a phosphor bronze cladding to create a highly corrosion-resistant composite, great for maintenance but at high initial cost. It's an exciting solution, if a little on the unproven side.

Ney and Partners / William Matthews Associates

I think this is both the best presented and the most spectacular entry to the contest. It looks like an arch, but it isn't. Two cantilevers project from either side of the gorge, leaving a tiny gap in the middle. It's highly poetic, yet structurally inefficient compared to a "proper" arch solution.

A slate deck sits on ultra-slender weathering steel plates, slung from the bedrock of Tintagel Castle and propped by weathering steel box girders below. Parapets, and a latticework between the girders, are in stainless steel.

The sense of insecurity inherent in the previous designs is redoubled here by the audacious midspan gap. I wonder how much that gap sways or drops when the bridge is eccentrically loaded by people or wind? The arrangement of the lower arch rib suggests it would be particularly likely to sway in high winds: it's widest at the centre of the span, but that's an arrangement which only makes sense if there's no gap.

All structural elements look impossibly slender at first sight, but I suspect they are not entirely implausible, with the latticework stiffening the lower "arch" box girder against buckling. The real problem with this design is the materials, as connecting stainless steel directly to weathering steel can induce corrosion in the latter, and even at this height, I suspect airborne salt spray would cause serious corrosion to the bridge over time.

Niall McLaughlin Architects / Price and Myers

Here's another design with an interesting use of materials, with a bronze balustrade upon a highly unusual prestressed granite bridge deck.

Prestressed granite is not a new concept: there have been quite a few bridges built in this way at a small scale, but not at the span proposed here. A larger bridge was proposed by German designer Heinz Hossdorf, but never built. The design here owes a debt both to Hossdorf and to Jurg Conzett. It would be built be cantilevering thin granite slices outwards from the supports, assisted by temporary cable support.

It would be an extremely durable solution, but the sheer weight of materials would render the construction phase expensive, certainly compared to some of the other designs. And, as with every design so far, the balustrade design may leave many users feeling exposed.

Here we have a second granite arch, although of more conventional design - a "traditional" masonry arch bridge, rather than a prestressed structure. Only the arch barrel is granite: the spandrel walls are of an unspecified stone, as are the balusters, which are a series of stone ribs with small gaps between them. The arch backing is a mix of foamed concrete with void formers, the latter to reduce weight.

The bridge would be erected on temporary centering, a very expensive business. The arch has a very low rise for a masonry arch, rendering it highly vulnerable to any ground movement, but there are few locations with such rocky ground to rely upon. The use of materials is inefficient, but if it works structurally, it's the most durable of all the solutions put forward.

I like the fact that the balusters provide a degree of physical and visual shelter, it's the only design which seems to me to really consider the experience of all possible visitors. However, I dislike its visual heaviness, particularly at the mainland end, where as well as being much deeper, the structure is also much wider. There could be considerable cost involved in stabilising the supporting rock slopes.

Wilkinson Eyre / Atelier One

The final design is in some ways the boldest proposal, an uncompromisingly modern structure, all high-tech and angular. It's a lightweight cable truss, with stainless steel struts and ties, and a composite deck comprised of oak and stainless steel elements. The composite construction renders the oak elements of the deck impossible to replace, so we'd have to hope they last as long as the rest of the bridge.

The bridge would be erected either on a scaffold or assembled from a temporary cable framework. The form of the bridge is such that is not functional until it is complete, hence the significant temporary works.

The bridge's main engineering challenge will be to ensure it is stable under pedestrian and wind loading. I wonder whether a less angular structure could have been attempted, and as with most of the designs, I think many users would feel insecure.

For me, none of these designs is the obvious winner, they are all flawed in different ways, and it's a case of choosing the least flawed option. I'd vote for the Ney design on sheer good looks, but would favour the Price & Myers design if it had a more sheltering parapet. It will be very interesting to see who wins, due to be announced in February.